It is known to apply a ceramic insulating material over the surface of a component exposed to gas temperatures that exceed the safe operating temperature range of the component substrate material. Metallic combustion turbine (gas turbine) engine parts (e.g. nickel, cobalt, iron-based alloys) are routinely coated with a ceramic thermal barrier coating (TBC).
The firing temperatures developed in combustion turbine engines continue to be increased in order to improve the efficiency of the machines. Ceramic matrix composite (CMC) materials are now being considered for applications where the temperature may exceed the safe operating range for metal components. U.S. Pat. No. 6,197,424, assigned to the present assignee, describes a gas turbine component fabricated from CMC material and covered by a layer of a dimensionally stable, abradable, ceramic insulating material, commonly referred to as friable graded insulation (FGI). Hybrid FGI/CMC components offer great potential for use in the high temperature environment of a gas turbine engine, however, the full value of such hybrid components has not yet been realized due to their relatively recent introduction to the gas turbine industry.
Combustor liners and transition ducts are gas turbine components that have a generally tubular shape defining an interior passageway through which hot combustion gasses flow. FIG. 1 is a partial perspective cut-away view of a prior art combustor 10, as described in U.S. Pat. No. 6,197,424. Such components have been formed by applying a layer of ceramic insulating material 14 to the inside surface of an annular CMC structural member 12. Such structures are difficult to manufacture due to their complex geometry, and in particular the difficulty of applying the insulating material 14 to the inside surface of the CMC structural member.
Existing methods of forming the insulating layer include casting or forming it directly to the CMC inside surface or fabricating the insulation material first and applying the CMC to the outer surface of the pre-formed insulation. In the former method, certain insulating layers such as disclosed in U.S. Pat. No. 6,197,424 require casting to thicknesses significantly greater than the final use required. This is due to the coarse grain structure, the need to cast to thicknesses 5-10 times thicker than the grain size to obtain uniform microstructures, and the difficulty in net shape casting of large thin shapes. Such thicknesses require excessive machining which may be difficult, costly, or impossible, depending on the shape. Furthermore, the large thicknesses present fabrication issues due to thick section drying and firing non-uniformities.
In the latter method, a certain amount of structural rigidity and strength are required in order to apply the CMC to the insulating layer. Typical insulating materials are quite porous (25-75% porosity) and are thus not strong or rigid enough for this purpose in their end use thicknesses (typically less than 5-8 mm thick). Thus, greater thicknesses are again required as above, with similar disadvantages as in the former method. Further disadvantage is encountered with large shapes, where forming a freestanding, self-supporting, and rigid structure becomes even more problematic (expensive tooling, detooling and handling issues, etc.)
The present invention addresses the above problems with alternative approaches, thus reducing the need for costly machining, forming of thick structures, forming of large, free-standing insulation structures, and the concomitant fabrication and handling issues.